The present invention relates in general to a system for dispensing a strand of fiber and pertains, more particularly, to an optical fiber payout system directed to the payout of an optical fiber substantially perpendicular to the axis of rotation of an fiber optic reel or bobbin associated with a weapon system or a communication system. The reel payout system of this invention is an improvement over the conventional longitudinal fiber payout systems.
With the conventional fiber dispensing systems it is necessary to provide payout of the fiber strand in such a way that strand stress is minimized. This is particularly important when the strand is an optical fiber used for communications purposes and particularly where such links are required to be established and maintained between relatively moving objects, such as, a launch platform and a launched weapon.
Fiber optic links are known for use between missiles or bombs and launch vehicles which are fixed (e.g., truck mounted launcher) and mobile (e.g., attack aircraft). Both the fixed and mobile applications guide the weapon in flight. However, it is recognized that difficult problems and complexities are associated with fiber optic communications links between two high velocity airborne vehicles moving relative to each other in an hostile military operations environment.
It is recognized that a fiber optic link between two vehicles moving at a high velocity relative to each other is susceptible to breakage, entanglement, and other operational and environmental stresses. These drawbacks can adversely effect the physical integrity, function, and performance of the fiber optic communications system. A significant drawback exists in a potential for the fiber optic cable to become entangled or break during pay out from conventional free-helix fiber optic payout devices.
Conventional free-helix pay out systems have numerous problems associated with the three stages of their existence. First, protecting the delicate fiber from moisture, dust, and physical damage prior to weapon release. Second, permitting fiber pay out after release without breaking it. Third, initiating pay out by releasing the fiber.
All of the foregoing drawbacks stem from a free-helix pay out in which the fiber leaves the launch vehicle travelling in a direction opposite the flight path of the launch vehicle. Furthermore, the fiber is on a cylindrical surface with the same direction as the bobbin, which intercepts the bobbin's axis at the peel point.
During fiber pay out the fiber motion and path is determined by fiber tension and aerodynamic forces. If, for any reason, the bobbin or any other interfering surface of the vehicle intercepts the fiber path, then fiber damage and failure of the data link is likely.
As a result, the bobbin must be at the rear end of the launch vehicle, facing directly aft and in line with the current flight path. If the launch vehicle experiences any significant angle of attack in the pitch or yaw planes, then the fiber risks probable breakage.
Prior to weapon release and payout a large circular hole through which a free fiber helix exits must be covered prior to weapon release. The exit hole must be covered in such a way as to protect the optical fiber data link and the remainder of the vehicle interior from all external sources of potential damage.
In general, the larger the circular hole, the more difficult it is to effectively cover. The entire perimeter of the hole must stay environmentally sealed over a wide range of operating and environment conditions. A six inch hole, for example has almost nineteen inches of perimeter over which a suitable seal must be maintained.
The cover referred to above must have the good seal instantly and reliably removed from the hole at pay out initiation. Cover removal must occur in such a way so as to pose no threat of damage to the fiber, the weapon, the launch aircraft, or other aircraft flying in formation with the launch aircraft. The requirement of rapid release in only a few milliseconds and the transition from a tight environmental seal to a complete release is a significant reliability problem.
The hole referred to above creates a point at which the aircraft can be observed by hostile forces. Whether before, during, or after flight, the large hole in the launch aircraft through which the fiber exits will likely have different dielectric behavior than that of the surrounding surface of the aircraft. This observable discontinuity will result in a reflection at radar frequencies which can be unacceptable for aircraft incorporating low-observable technology.
Another drawback associated with fiber optic pay out systems is the problem of housing the cable in order to allow simultaneous pay out at a high rate of speed between a moving launch aircraft and a weapon released from the aircraft toward a distant target.
The optical fiber is normally stored and transported on spools or bobbins having a generally cylindrical shape or a tapered cone-like shape. The optical fibers are typically wound in a tight, closely packed helix about the outside diameter of the bobbin. When the optic fiber is dispensed, or paid out from the spool on the bobbin at high speed, it is known to pull the optic fiber off the bobbin in a direction generally parallel to a longitudinal axis of the bobbin and toward the small or truncated end of the cone.
It is known and frequently observed that if the optic fiber is wound on the bobbin in a helix pattern and then pulled from a point distant from the small end of the bobbin and along the longitudinal axis, then the optic fiber leaves the bobbin in the form of a helix. The helix of optic fiber has a helix diameter that gradually decays from a diameter approximately equal to the bobbin diameter to a diameter of essentially zero. The decay typically requires the pay out of optic fiber equal in length to many hundreds of bobbin diameters.
The form of the substantially unrestrained or free-helix and the helix decay rate are functions of the geometry of the bobbin and the fiber. Other factors include the bulk material properties (e.g., fiber density and stiffness are properties of primary concern), the coefficient of friction of the optic fiber on itself, and the drag characteristics of the fiber in the respective medium (e.g., air) through which it is paid out.
In many applications the fiber must be constrained to a helix diameter which is relatively small in comparison with the bobbin diameter, through a pay out length shorter than that typically required for the natural decay of the free-helix. This is normally accomplished by guiding the fiber through a conventional rigid guide ring having a desired inside diameter and oriented in a plane perpendicular to the axis of the bobbin and also concentric with this axis.
A serious drawback of these pay out systems is that the physics of the pay out dictate that the conventional rigid guide ring constrains the helix with a resulting tendency of the optic fiber helix to "balloon" resulting in the swelling of the helix diameter before the optic fiber has passed through the conventional guide ring. The swelling takes the shape of a smooth curve outward from the point where the optic fiber leaves the bobbin and before curving down to the diameter of the constraining ring.
The shape of the aforementioned curve is determined by the above indicated factors, the diameter of the conventional guide ring, and the distance from the conventional guide ring to the point where the optic fiber leaves the bobbin. Increasing the pay out velocity increases the maximum diameter of the "balloon". This increases the tensile and bending loads placed on the optic fiber as it passes through the conventional guide ring.
Yet other drawbacks to conventional fiber optic pay out systems include two practical velocity limitations in the actual application and use of conventional pay out systems. One limitation results from the tendency of the optic fiber to balloon as the velocity increases resulting in potential interference between the optic fiber and adjacent, fixed components of any associated fiber optic pay out mechanism.
Another limitation is the actual loading on the fiber itself of static and dynamic stresses as it passes through the conventional guide ring. These loads may eventually reach the strength limit of the optic fiber at a particular velocity resulting in fiber failure. The bending loads on the optic fiber are a particular concern for the optical fibers of a weapon system. Optical fibers are typically stiff, brittle mono-filaments.
It will be recognized by those skilled in the art that the limitations related to optical fibers are applicable to mono-filament strands in general as they are paid out through a constraint such as a guide ring. It will be understood from the objects and description of the present invention that this invention is readily applicable to solving equivalent problems in the bobbin, textile, and wound fiber art.
It is believed that the similar problems encountered in the textile industry with conventional textile fiber pay out mechanisms, the pay out mechanisms used in the manufacture of filament wound structures in general may use a mechanism similar to that set forth in the following specification and claims. Similar problems are believed to exist in the telecommunications industry relative to the dispensing of optical fiber for communications purposes.
However, for the purposes of clarity and precision, the described embodiment will be limited to a launch platform (e.g., aircraft) and weapon combination.
Two additional stress problems related to the free-helix pay out of mono-filament strands can occur if the natural helix decays rapidly enough such that the bobbin structure prevents the optic fiber from following that helix. These conditions can occur when the mono-filament strand pay out is from the end of the bobbin furthest from the direction of pull.
One stress problem has been observed to occur when the taper angle of the conical bobbin is less than the decay angle of the pay out helix. The optic fiber tends to wrap tightly around the bobbin, drag along the bobbin, and pull sharply in the direction of the longitudinal axis at the end of the bobbin.
Another problem occurs as a result of conditions in which the axis of the bobbin is not aligned with the pay out direction and the bobbin does not include the guide surfaces set forth below. In this condition the optic fiber is trying to pull its way through the bobbin along the bobbin surface farthest from the pay out direction. The optic fiber is forced to drag across the corner of the bobbin.
Approaches to solving the foregoing and other problems are all directed to a dispensing system wherein the fiber strand is wound on a bobbin and the bobbin is mounted in a housing or a canister along and generally parallel to the longitudinal axis of the housing or canister. The optic fiber in the foregoing approaches still provides for optic fiber payout generally parallel to the longitudinal axis of the bobbin.